Fish mouth fitting or
“coping” tube for motorcycle frames is pretty easy with a low cost tube-cutting
jig.Below is a picture of an import
version of this type tube cutter. The base angle was missing so a cheap vise is
used to fasten the jig to the drill press table. You can use these jigs without
a drill press and use a drill to power the saw. Using the drill press makes the
jig and saws last longer. Hole saws are used to make the actual cut. The tube
is held in a clamp in the jig and the clamp can be set to various angles for
angle cuts.

Here is a shot of the saw
part way through the tube showing progress of the cut.

After you cut through the
tube the end of the tube looks like the picture below. The tube is pretty
worthless for welding at this point. Note the rough and sharp thin edges.

Next, the tube is sanded
(disc or belt sander) or filed (half round file) so the sharp edges are squared
off back to the tube wall thickness. Then the tube is beveled to allow the weld
to fully penetrate. The bevel allow the weld to start at the inside diameter of
the tube to the outside of the tube. The joint is very tight so there are no
gaps around the inside of the tube where it meets the other tube. A tight fit
protects the root of the weld from air contamination that leads to oxidation,
bubbles or inclusions in the weld.

Tubes are fitted together
and tacked in several locations around the tube. Short welds are done to
prevent distorting the frame. Also, this is probably one of the hardest welds
to do since there are no straight lines and position changes constantly
throughout the weld.After you do
several sample welds and can make the weld look good on the outside, it is time
to cut some apart and really find out how good they are.In the picture below, a “Tee” sample was cut
though the centerline of the tubes. You can see the outside of the weld fillet
where the two tubes were welded from the outside. There is no gap at the inside
of the tube and the weld has completely fused the two tubes together. If the
sample was polished, you would be able to see the actual weld which is a bullet
shape starting from the outside fillet to the inside diameter of both tubes
without burning through either tubes.

Weld that burns through the
tube at the cut end or sidewall will be exposed to air and can cause issues
with the weld strength. Ideally, the weld will just barely penetrate the tube
and not be allowed to pick up oxygen. The welding process shield protects the
outside of the weld. A sample cut like the picture above lets you see the
inside of the tube, weld fillet, weld penetration, weld porosity or inclusions
and wall thickness of the tube. Be sure the weld has not consumed part of the
tube wall thickness or strength will be lost. The tube wall thickness should
not be thinned near the weld (also called undercutting).